This invention relates to a modular digital telephone system utilizing time division multiplex switching, and more particularly, to such a system having distributed architecture with fully distributed switching and control a distinguished from centralized processing where all call processing is directed through a centralized point.
Typical PBX telephone exchanges are generally characterized with central processing and in which the number of telephone handsets exceeds the number of outside lines. Accordingly, handset users may be blocked from making outside calls until a particular trunk is free and the call put through. In U.S. Pat. No. 4,597,077, this situation is somewhat alleviated by utilizing a distributed star architecture which employs peripheral switching units (PSU) connected to a central switch or location, and further to employ time division multiplex switching in which communication lines are assigned to each device connected to the line for a relatively short period of time, referred to as time slots. The time slots occur periodically and are repeated at a frequency such that a device attached thereto can send or receive data continuously at a given data rate. A frame is comprised of all of the time slots available for the devices connected thereto.
Digital systems employing time division multiplex in a PBX of the type described in the aforesaid U.S. Pat. No. 4,597,077 still require a central processing unit (CPU) which assigns time slots dynamically. For example, assuming that there are 20 to 100 extensions for each PBX trunk (outside line) each phone call from an extension has time slots assigned dynamically upon call initiation. If the time slots have all been assigned when a call is requested, that particular call is blocked until a time slot becomes available. In addition, of course, the centralized processing provides the status of each line and the extension to all of the consoles which are connected into the system. If something goes wrong in the CPU, the entire system is shut down until the problem in the CPU is located and corrected.
In modular telephone construction, for example, the ViAX instant voice communication system manufactured by the assignee of the present invention, all of the circuitry and electronic components are incorporated on modular cards of integrated circuits which are mounted and interconnected in racks in order to conserve space as well as to provide short interconnecting lines. Accordingly, the telephone system is compactly packaged and configured in such a way as to require limited back room equipment space. The use of the modular cards also allows great flexibility in the expansion and/or configuration of the local telephone system. The system is configured through an external multiple access programable computer and information system data processor. With proper monitoring, the individual cards can be diagnosed for trouble as well as provide redundancy so that if one of the cards should malfunction, repair is simplified by simply replacing the faulty card in the system. However, with central processing, a malfunctioning CPU could cause serious problems in the functioning of the system. Furthermore, in a central controlled environment, the monitoring of all the states of the lines in the system to indicate their condition at any time to a console in the system places a heavy burden on the CPU.